1. Field of the Disclosure
The present disclosure relates generally to motor speed controls, and more particularly to those used for toner metering devices.
2. Description of the Related Art
In electrophotographic imaging apparatus, motor driven toner metering devices are used to convey toner from a toner reservoir for use in the imaging unit of the imaging apparatus. DC motors are typically used to drive toner feed mechanisms. A brush DC motor can provide enough torque and speed range to drive the toner metering system. Accurate rotational speed control of these motors ensures reliable operation of the imaging apparatus. To achieve good speed control, a way to measure the motor speed accurately is needed. There are several methods to measure a speed of a brush DC motor. One could mount an encoder, such as an optical or magnetic encoder on the motor shaft in such a way to produce pulses as the motor shaft rotates. These sensors provide highly accurate information about the motor speed, however, they add about cost in material and in assembly.
One example speed control system uses a brush DC motor with a single channel encoder mounted on the back. However a single channel encoder cannot tell the direction of the motor rotation so an additional sensor is needed to determine rotational direction of the toner metering device. Also with a single channel encoder any signals generated from vibrations or electrical noise will cause the accurate information of the toner metering mechanism to be lost. A quadrature encoder which provides information about direction as well as speed could be used. However, the quadrature encoders are expensive. A stepper motor, which advances one step for each step command, allows control of exact motor position. This solution could thereby eliminate the need for the additional directional sensor. One potential problem is that when the stepper motor is turned off, there is no guarantee that the stepper motor can start at the same position, leading to some uncertainty about the position of the toner metering device. Stepper motors are also prone to acoustical noise and excessive power consumption due to their extremely inefficient operation.
An alternate way to measure motor rotational speed is to use the back EMF feedback from the DC motor. It is known that when a brush DC motor turns, it generates a voltage across its winding. The voltage is called the back EMF voltage and it is proportional to the rotational speed of the motor as shown by the following equation;Vemf=Ke˜ω[Volt]  Eq. 1where Vemf is the back EMF voltage in volts, Ke, is the back EMF constant of the motor in volt-s/rad, and ω is the rotational speed of the motor in rad/s. The motor speed can be estimated by measuring the motor back EMF voltage and dividing it by the back EMF constant Ke. Motor speed control can be performed using the measured speed from the back EMF feedback. The back EMF speed control is an economical solution because it does not require any sensors or encoders on the motor and the electrical sampling circuitry for sampling or measuring the back EMF voltage Vemf is quite simple. However, one drawback of the technique is that the measured speed is only as accurate as the estimation of the back EMF constant Ke. The back EMF constant Ke could differ as much as +/−15% from motor to motor at the beginning of life, and +/−25% over the life of a motor. For some applications, this amount of error in motor speed is unacceptable.
It would be advantageous to be able to perform a back EMF feedback calibration to periodically update the back EMF constant Ke without having to use encoders on the motor. It would further be advantageous to use for the calibration input from an existing sensor in the toner feeding mechanism.